A. Propeller; B. Motor (partly hidden by shield); C. Pilot’s seat; D. Sustaining plane; E. Rudder; F. Elevating-plane; G. Chassis.] This scouting biplane is intended for detailed reconnaissance; for a careful flight above an enemy’s position, that is to say, in which every movement of troops is observed and noted. Another machine becomes necessary for the making of a quick and general survey--such, for instance, as that in which a pilot would be asked to fly to some definite point, and see whether a body of troops is stationary or on the march. Speed, for such a task, is the chief requirement. Hence there is a type of fast scouting monoplane, in which a pilot can ascend alone, and fly at 100 miles an hour. With such a craft, sweeping rapidly above an enemy’s position, the pilot-observer can return with his information at surprising speed. In Fig. 61 an air-scout of this type is seen. The tapering, covered-in body will be observed; this is to reduce wind resistance as the machine rushes through the air. The Gnome engine is, for the same reason, covered by an aluminium shield, which only allows the lower cylinders to project; they must, of course, be exposed in some way to the air, or they would not cool themselves. The landing-carriage has been reduced to its simplest form; this, again, is to reduce wind resistance; and the pilot, sitting deep in the body, shows only his head as the machine flies. Here, again, apart from the greater comfort in being so shielded, the placing of the pilot within the machine spells a lessening of pressure. This question of the resistance of the air is vital in the designing of high-speed craft. Every projecting surface must be “stream-lined,” as it is called; it must, that is to say, have such a tapering shape as will offer least resistance as it passes through the air. The atmosphere is viscous--or, to use a commoner word, “sticky.” It clings to anything that is passed quickly through it, as treacle might adhere to the back of a knife; therefore the body of an aeroplane--like the hull of an airship--slopes away to a point astern, so that friction may be minimised. What the pressure of the wind may mean, when high speeds are reached, one may prove by holding a hand from the window of an express, say while it is running at 60 miles an hour. The rush is so fierce that the arm will ache from the mere effort of holding it outstretched. Whereas the train is running at 60 miles an hour, a monoplane may be flying at more than 100 miles an hour. Therefore the thrust of the wind against wires, struts, and body may be imagined; it reaches the violence of a hurricane. Even the pilot, glancing above the wind-screen to steer his course, finds the pressure so tremendous that, in some racing machines, he is provided with a padded rest against which he can lean his head.